In general, a spark plug includes an outer shell and an insulative core. At a firing end of the spark plug, a firing electrode extends from the insulative core and a ground electrode extends from the outer shell. The two electrodes define a spark gap between them. In a combustion engine, a spark formed in the gap is used to ignite a mixture of fuel and air.
In conventional two- and four-stroke combustion engines, a combustible fuel from a fuel injector or carburetor is mixed with air in the intake. When the intake valve or port opens, a fuel vapor, consisting of fuel and air, is pulled by negative pressure into the combustion chamber where it dissipates throughout the chamber. A spark at the spark plug ignites the fuel vapor, causing rapid expansion of the chamber contents and corresponding displacement of a piston.
A recent modification to the conventional combustion engine is the gasoline direct injection (GDI) engine. A defining characteristic of the GDI engine is the injection of fuel as a liquid. A second defining characteristic is the introduction of fuel and air into the combustion chamber through separate ports. Thus, rather than injecting a fuel vapor consisting of fuel and air, the combustible fuel is injected under pressure as a liquid through a fuel intake port and air is introduced through an air intake port, both of which open into the combustion chamber.
The fuel is sprayed as a liquid directly in the direction of the spark gap. The spray spreads out from the fuel intake port in the shape of a cone (i.e., a spray cone) with the central longitudinal axis of the spray cone aimed at the spark plug gap. A small percentage of the fuel vaporizes and mixes with the air injected into the chamber through the air intake port. By aiming the central longitudinal axis of the spray cone directly at the spark plug gap rather than attempting to fill the entire chamber with a combustible fuel vapor, less combustible fuel is needed to create a suitable displacement of the piston. A spark ignites the fuel during the brief period in which the fuel surrounds the spark plug gap, rather than at a later period in which the fuel would have dispersed to fill the entire chamber. Reduced fuel usage also results in a reduction in polluting emissions.
In GDI engines, the fuel is injected into the combustion chamber at different stages in the compression stroke depending upon speed. At low speeds the fuel is injected late in the stroke while at high speeds the fuel is injected early in the stroke. At low speeds, because of the high compression in the chamber resulting from the late stage of the compression stroke, the cone of fuel sprayed at the gap does not rapidly disperse throughout the chamber. Instead, the flow pattern of the spray cone is not substantially altered as it moves toward the gap. At high speeds, the fuel is injected when there is less compression and different flow patterns in the chamber. These differences cause the fuel to mix throughout the chamber.
In two-stroke GDI engines the fuel is injected during each cylinder compression stroke whereas in four-stroke GDI engines the fuel is injected during alternating cylinder compression strokes.
Although a direct injection (DI) engine may be operated using gasoline, as described above, other combustible fuels, such as alcohol, may be used. Moreover, while the combustible fuel may be injected in the combustion chamber in the form of a spray cone of fuel, it also may be injected as a stream of fuel or in other variations or shapes.